It is known that a primary requirement concerning the wires of this type is, in addition to elevated mechanical characteristics (in particular tensile strength), good hydrogen-embrittlement resistance in sulfide-containing acid media, in particular in the form of H2S present in the fluids and hydrocarbons being transported.
It is recalled that this resistance is the subject matter of NACE and API standards, particularly:
the NACE TM 0284 standard for the resistance to cracking by hydrogen or “HIC” (Hydrogen Induced Cracking) in seawater saturated with acid H2S;
the NACE TM 0177 standard for the resistance to cracking under H2S stress, or “SSCC” (Sulfide Stress Corrosion Cracking) in acid media. For the use under consideration here, it is imperative that the profiled wires must now satisfy this, in view of increasingly more difficult operating conditions (great depth);
and the API 17J standard (Specifications for unbonded flexible pipes) for the evaluation of the HIC and SSCC resistances on the basis of a stress test in an acid medium.
These profiled wires may have a round cross section, obtained by simple drawing starting with a wire rod of larger diameter. They may also have a rectangular section after drawing, rolling or drawing followed by rolling, or may be profiled with U-shaped, zeta or teta cross section, etc. in such a way that they can be interlocked with one another along their edges or be joined by folded seams to form articulated reinforcing laps.
Today, the commercial products available in the field of steel wires of NACE quality for offshore use lie mainly in low-alloy steel grades ultimately capable of a final tensile strength (Rm), therefore after quenching and annealing, of approximately 900 MPa.
These profiled wires are usually manufactured in known manner by using carbon manganese steels containing 0.15 to 0.80% C (by weight) and initially having pearlito-ferrite structure. Traditionally, after the initial round, rolled wire rod has been profiled, it is subjected to appropriate stress-relief heat treatment to obtain the required hardness. It is by virtue of this hardness that the nominal criteria for use are respected, for example the ISO 15156 standard, which stipulates that these Mn steel grades must have a stress resistance in H2S media suitable for the “profiled wire” use in question here, if the wire hardness is lower than or equal to 22 HRC.
However, the profiled wires obtained by the traditional methods have the reputation of being poorly able to resist the relatively severe acidity conditions encountered in deep waters, those provided for by the NACE TM 0177 standard with solution A (pH 2.7 to 4) in this case, due to the concentrated presence of H2S in the hydrocarbon being transported, all the more so if the targeted hardness levels are greater than 28 HRC (greater than 900 MPa).
This is undoubtedly also the reason for which the document PCT/FR91/00328, published in 1991, describes a thermomechanical method for producing a profiled wire of pearlito-ferritic structure that has a carbon content of between 0.25 and 0.8% and that satisfies the NACE TM 0177 and TM 0284 standards with solution B (pH 4.8 to 5.4), albeit at the cost of final annealing, which relaxes the mechanical strains imposed by work-hardening of the metal and thus lowers the tensile strength (Rm) to approximately 850 MPa.
The document FR B 2731371, published in 1996, also relates to the production of profiled wires of carbon steel for reinforcement of flexible offshore pipelines whose resistance to acid media containing H2S is sought at a high level on the basis of general knowledge about the influence of steel microstructures on its resistance to hydrogen-induced embrittlement. The profiled wire proposed in this document, containing 0.05 to 0.8% C and 0.4 to 1.5% Mn, has been subjected after forming (drawing or drawing and rolling) to quenching followed by final annealing. The metal structure obtained is substantially an annealed martensitic bainite. In this way, profiled wires ready for use would be obtained, which wires would have elevated mechanical characteristics, i.e. an Rm close to 1050 MPa (therefore in a quenched and tempered steel to attain hardness levels as high as 35 HRC, but observed in the industry to be closer to approximately 820 MPa) and consequently would be able to clearly exceed those recommended by the ISO 15156 standard, and would be resistant to very acidic media (pH close to 3). It is stipulated therein that, in the absence of final annealing, a wire can be obtained that has superior hardness along with even higher mechanical characteristics, although consequently with clearly less chemical resistance to acid media.
In fact, it is found that the characteristics of very high level that are usually required of such wires actually have to be satisfied only in a limited number of cases of use.
In agreement with the NACE quality, a resistance in conformity with the aforesaid API 171 standard, with an H2S partial pressure that may attain 0.1 bar and with a pH of 3.5 to 5, would actually be sufficient to cover the essentials of the effective needs, whereas the profiled wires manufactured by the method according to the document mentioned in the foregoing have what we might call over-qualified resistance, because they meet the elevated requirements of the TM 0177 and TM 0284 standards established with solution A, having a pH of approximately 3.
Furthermore, it turns out that the usual profiled wires on the market, with pearlito-ferritic structure without final heat treatment, are unsuitable most of the time for satisfying even the moderate NACE requirements.
In addition, since flexible offshore pipelines are being called upon for use at progressively greater submersion depths, a demand is now actually developing in favor of strengths further increased by several hundred MPa, in order to attain, shall we say, strengths on the order to 1300 MPa and even higher, without in turn degrading the NACE quality, while it must be recalled that embrittlement of the steel by hydrogen-induced corrosion and mechanical characteristics are opposing properties: seeking to favor one is doing so to the detriment of the other, and vice versa.
In addition, steadily increasing market pressure is being felt on the prices, with the consequence of greater than the usual recourse to noble alloying elements, such as chromium, niobium, etc., or long or multiple and therefore costly treatment steps, especially if they must be carried out at high temperature.
In this regard, particular note will be made of the teaching of JP 59001631 A of 1984 (DATA BASE WPI Week 198407 Thomson Scientific, London, GB; AN 1984-039733), which recommends a final long-duration recovery treatment of the wire, in the form of annealing for several hours.
Similarly, the method described in EP 1063313 AI imposes very high work-hardening ratios of the wire, close to 85%, to achieve the desired final diameter by drawing.
Note also will be made of the existence of EP 1273670 relating to the manufacture of steel bolts, but wherein the teaching emphasizes the advantage that may be expected in the corrosion resistance under tension of pearlitic bolts.